1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and more particularly to an electrophotographic photosensitive member comprising a low-molecular organic photoconductive material that can impart improved electrophotographic characteristics.
2. Related Background Art
Various types of organic photoconductive polymers including polyvinyl carbazole have been hitherto proposed as photoconductive materials used in electrophotographic photosensitive members. These polymers, however, have been put into practical use with difficulty up to the present, notwithstanding their superiority to inorganic photoconductive materials in respect of film-forming properties and lightness in weight. This is because they have achieved no sufficient film-forming properties and also are inferior to the inorganic photoconductive materials in respect of sensitivity, durability, and stability against environmental changes. Also proposed are low-molecular organic photoconductive materials such as hydrazone compounds as disclosed in U.S. Pat. No. 4,150,987, triarylpyrazoline compounds as disclosed in U.S. Pat. No. 3,837,851, and 9-styryl anthracene compounds as disclosed in Japanese Patent Laid-Open Application No. 51-94828 and Japanese Patent Laid-Open Application No. 51-94829. Such low-molecular organic photoconductive materials have become able to eliminate the disadvantage in film-forming properties that has been questioned in the field of organic photoconductive polymers, by appropriately selecting binders to be used. They, however, can not be said to be sufficient in respect of the sensitivity.
Under such circumstances, a laminated structure comprising a photosensitive layer separated into a charge generation layer and a charge transport layer has been proposed in recent years. Electrophotographic photosensitive members having the photosensitive layer of this laminated structure have become able to improve the sensitivity, charge retension, surface strength, etc. to visible light. Such electrophotographic photosensitive members are disclosed, for example, in U.S. Pat. Nos. 3,837,851 and No. 3,871,882.
However, in the electrophotographic photosensitive members employing the conventional low-molecular molecular organic photoconductive material in the charge transport layer, although the sensitivity and properties thereof have reached a practical level, no sufficient solution has been achieved for the disadvantage that light portion potential and dark portion potential may greatly fluctuate when charging and exposure to light are repeatedly carried out. To suppress this fluctuation of potential during repeated use, Japanese Patent Laid-Open Application No. 57-122444 and No. 62-39863, for example, disclose a method in which an antioxidant is mixed into the charge transport layer, and besides, Japanese Patent Laid-Open Applications Nos. 53-26128, No. 60-164745, No. 62-105151, etc. teach a method in which a stabilizer is added to effect stabilization. These methods, however, are presently under the conditions that no sufficient effect can be achieved particularly when the photosensitive members are repeatedly used over a long period of time.